Graphene is a one-atom-thick planar sheet of bonded carbon atoms that are densely packed in a honeycomb crystal lattice. Graphene can be visualized as an atomic-scale two-dimensional wire mesh made of carbon atoms and their bonds. Since the reports of its first isolation and the observation of the quantum Hall effect, single layer graphene (SLG) has attracted intense research efforts both from academic and industrial communities. Based on the discovered properties and behaviors of SLG, the possible incorporation of SLG into devices such as integrated circuits offers potential benefits in terms of further miniaturization, increased operating speeds, lower power requirements, and reduced operating temperatures.
Graphene can be produced through mechanical exfoliation of graphite. In one example of an exfoliation method, graphite is repeatedly exfoliated using scotch tape to remove increasingly thinner graphite crystals until graphene is eventually adhered to the tape. While the original exfoliation method led to many exciting discoveries regarding this unique two dimensional atomic crystal, exfoliated graphene has unfortunately proven difficult to integrate into large scale production of conventional electronic, mechanical, and optoelectronic circuitry.
Several alternative synthesis methods have been reported to try to produce graphene with desirable physical properties similar to those of exfoliated SLG, with minimal spatial variation, over extended areas. These alternatives include epitaxial growth of graphene out of silicon, reduction of graphene oxide, direct growth of graphene onto thin nickel film, and most recently, direct growth onto copper foils. Among these methods, the copper foil-based synthesis has been the most effective in producing large areas of continuous SLG with promising electrical properties including a carrier mobility of ˜4000 cm2/V·s. The nickel based synthesis also showed excellent physical properties, most notably the quantum Hall effect, and it has been shown that further optimization leads to the formation of 87% single or double layer graphene in area. Unfortunately, using these graphene materials for device applications requires a transfer step because the growth substrate is not compatible with device fabrication procedures. Furthermore, this extra transfer step for depositing synthesized graphene onto a device substrate can cause a number of significant problems. First, the large area SLG is mechanically delicate can easily be damaged during the transfer, resulting in a high defect rate. Second, alignment between the graphene film and the target substrate presents additional technical challenges potentially resulting in the need for increased tolerances that sacrifice device density and/or can result in increased defect rates. Third, these transfer procedures are often performed in aqueous solutions and it is difficult to remove the liquid residue trapped within the interfacial space between graphene and the target substrate, resulting in unwanted spatial variation and still further defects.
Therefore, it would be desirable to have a reliable, transfer-free method for batch fabrication of single layer graphene devices with physical properties similar to those of exfoliated SLG, and with minimal spatial variation over extended areas.